Acceleration cancelling hydrophone



y 1969 o. E. HANCKS ET AL 3,458,857

ACCELERATION CANCELLING HYDROPHONE Filed Oct. 12. 1967 ns-4 OP INVENTORS n 4 MW 3 Hmw n Y M 7 HMA/k mu N m 4 oaw,

United States Patent 3,458,857 ACCELERATION CANCELLING HYDROPHONE Dorothy E. Hancks, William C. Hubbard, and Claude C. Routh, San Diego, Calif., assignors to the United States of America as represented by the Secretary of the Navy Filed Oct. 12, 1967, Ser. No. 674,990 Int. Cl. H04b 13/02 US. Cl. 340- 2 Claims ABSTRACT OF THE DISCLOSURE In a towed hydrophone, one piezoelectric element is compressed and another element is tensioned by acceleration in the direction of the towing cable. The voltages thus produced are out-of-phase and when added tend to cancel. Both elements, however, respond normally to the pressure waves producing in-phase voltages.

Statement of Government interest The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

Background Underwater listening devices carried by a moving boat can be streamlined to minimize turbulence and local noise, but to remove the hydrophone from the machinery and propeller noises, long towing cables have been used to permit the hydrophone to travel astern. A second order eflect now arises. A whipping motion appears in the cable and uncontrollable accelerations at the hydrophone occur. The resulting electrical noise can be many decibels above the level of the wanted signals.

The object of this invention is to provide an improved hydrophone.

A more specific object of this invention is to provide a hydrophone which is relatively insensitive to acceleration pressure in one direction while being quite sensitive to normal pressure waves of wanted signals.

Summary The objects of this invention are obtained by so mounting two groups of piezoelectric elements that acceleration in one direction places one group in compression and the other group in tension. When the electrical terminals are all connected in parallel, the differential voltages tend to cancel while the normal pressure waves of wanted signals produce in-phase electrical signals.

Other objects and features of this invention will become apparent to those skilled in the art by referring to the preferred embodiment described in the following specification and shown in the accompanying drawing in which:

FIG. 1 is a elevational view of the hydrophone assembly embodying this invention,

FIG. 2 is a detailed, partly-sectioned view of the hydrophone of FIG. 1, and

FIG. 3 is a circuit diagram of the connections of the plural piezoelectric elements of FIG. 2.

The typical application of the hydrophone of this invention illustrated here is that where the hydrophone is mounted on the outer end of a long towing cable. In FIG. 2 the cable is shown at 1, the end of which is terminated in the streamlined body and 22. Preferably, the sensitive piezoelectric elements are mounted in the forward compartment of the body at 20 whereas preamplifiers are mounted in the aft portion of the body 22.

The towing cable 1 is preferably of the type comprising plural pairs of conductors enclosed in a woven steel ice mesh or sheath. The whole being impregnated with rubber or rubber-like, water-tight coating.

The strands at the end of the steel portion of the sheath shown at 2 in FIG. 1 are unraveled and are joined by welding or brazing to the forward end of the metal spool 3. The spool is machined from bronze or other non corrosive metal and is turned with an outwardly extending flange 4 intermediate the ends of the spool.

Two groups of piezoelectric elements are mounted on the spool. The first group comprising rings or cylinders 10 and 11 of polarized ceramics, such as barium titanate or lead zirconate, which are quite sensitive to pressure and produce voltages across metal electrodes deposited on the inner and outer surfaces of the rings. Piezoelectric elements 10 and 11 are larger in diameter than the spool 3 and are insulated and spaced therefrom by rubber rings 5, 6, and 7. Each ring is fashioned with a shoulder to hold the rings 10 and 11 in concentric spaced relation. Fine wires, not shown, contact the electrodes deposited on the inner and outer electrodes and are conducted through the spool and hence, into the cable 1 at one end or if desired into an amplifier in the casing 22 at the other end.

The second group of piezoelectric elements 12 and 13 are in all respects like elements 10 and 11 and are also ring shaped and are supported on the right hand side or aft side of flange 4. The rings 12 and 13 are spaced concentrically about the spool by insulating spacer rings 15, 16 and 17. Elements 12 and 13 likewise have deposited metal electrodes on the inner and outer surfaces and are connected by lead wires to the interior of the spool 3.

As stated, the object of this invention is to cancel the voltages caused by lengthwise accelerations of the hydrophone, and to this end the two groups of piezo elements are, respectively, simultaneously compressed and tensioned to produce equal and opposite voltages. The magnitude of either voltage is a function of the distortion of piezoelectric elements which in turn is a function of the magnitude of acceleration and the magnitude of the mass opposing the acceleration. Assuming that positive-going and negative-going voltage changes at the element terminals are linear functions of compressional forces and tensional forces, then the mass forward of one element group should be approximately equal to the mass aft of the other group. These simple relationships will of course change when friction and mechanical impedances in the system are added. Without lessening the generality of this invention, it will be assumed that the principal force, because of acceleration, is applied at the forward and aft surfaces of flange 4. The flange 4, in FIG. 1, will push piezoelectric elements 10 and 11 and will pull elements 12 and 13. In such an arrangement, the end-to-end elements 12 and 13 must be joined together and to the aft surface of flange 4. Conveniently, elements 12 and 13 and spaces 15, 16 and 17 may be bonded with a strong adhesive and the left spacer, 15, glued to the flange.

Ring 18 is slipped onto the spool at the right hand end and is cemented to the collar 19 which in turn is attached to the casing 22 containing the amplifier.

In the specific embodiment shown, the spacers 5, 6, 7, 15, 16, and 17 as well as collars 18 and 19 are preferably loosely or slip-fitted over the spool 3. The entire assembly must be protected from sea water. To this end the assembly forward of the amplifier case 22, is encased in a semi-soft jacket 20 of rubber-like material, such as neoprene which may be either precast or molded in place. The rear end of jacket 20 is cast around ring 19 and into openings 19a and 22a in the ring and casing 22, to positively interlock the aft end of the jacket with the ring assembly 19, and with the amplifier case 22. Attachment between to case 22 and jacket 20 may be screw threads or adhesives. Now, the lengthwise distortions of the piezoleft with the same acceleration. Such acceleration presses 1 flange 4 against the end of spacer 7 and tends to compress the piezoelectric rings 10 and 11. The same movement of flange 4 pulls on spacer 15, and tends to stretch the attached rings 12 and 13. If compression of rings 10 and 11 produce an increased voltage across the electrodes, the tension of rings 12 and 13 produce a decreased voltage across the electrodes. With corresponding terminals of the electrodes connected in parallel as suggested in FIG. 3 the positive-going and negative-going voltages tend to cancel and the input to the amplifier receives little or no voltage caused by the acceleration.

Pressure waves orthogonal to the tow axis impinging upon the hydrophone 20, however, produces the same distortions in all of the piezoelectric elements 10, 11, 12, and 13. Hence wanted signal voltages of all elements are added, thus, materially increasing the signal-to-noise ratio at the terminals of the hydrophone.

What is claimed is:

1. A hydrophone assembly comprising;

two groups of piezoelectric elements, each element having electrodes which produce a signal voltage when the body of the element is subjected to pressure from either of two orthogonal directions,

a spool, means connected to said spool to axially accelerate said spool in one of said orthogonal directions, a circumferential flange aflixed to said spool inter-mediate the ends of said spool,

said elements comprising rings of piezoelectric material, said rings comprising said two groups, being slidably telescoped over opposite ends of said spool so that acceleration of said flange compresses the rings of one group while tensioning the rings of the other group,

the electrodes of said elements of both groups being connected to additively combine the signal voltages of all elements,

said two groups of elements being so disposed as to be similarly distorted by a pressure wave from the other of said orthogonal directions for producing wanted in-phase voltages, and to be differentially distorted by the acceleration pressure from the other of said directions for producing cancelling out-of-phase voltages.

2. The hydrophone assembly defined in claim 1 further comprising spacers of insulating material attached to and yieldably supporting said rings concentrically about said spool so that compressing and tensioning pressures from said flange are transmitted, respectively, to said two groups.

References Cited UNITED STATES PATENTS 3,187,300 6/1965 Brate 340-10 CHARLES L. WHITHAM, Primary Examiner 

